AWS: Please switch to newer AMI's

The docker machines default AMIs are Ubuntu 16.04 LTS build 20180228.1; Which comes with a rather outdated 4.4 kernel

In particular I ran into issues when using jobs using docker:dind for kitchen-ci with kitchen-dokken as the cgroups in the spawned test docker weren't setup correctly. Switching to the Ubuntu 20.04 LTS AMI (ami-0ffac660dd0cb2973 in the eu-west-2 region).

Given the amount of changes in the last 4 or so years in cgroups and the likes that's not too surprising that corner-cases like the above have surprising behaviour. Hence it would be good to switch to more modern base AMIs